Liquid dielectric cooled terminations

ABSTRACT

A termination for terminating or absorbing a signal transmitted along a transmission line is disclosed as comprising a short circuit disposed across the transmission line and a thin film of a resistive material disposed a critical length from the short circuit to absorb substantially the signal transmitted along the transmission line. A cooling chamber is formed about the thin film and a cooling liquid of given dielectric constant and relatively low microwave loss is directed through said cooling chamber to remove the heat absorbed by the thin film. A window is disposed across the transmission line to form the cooling chamber. The cooling liquid of given dielectric constant acts as a matching transformer to reduce the impedance seen by a wave propagated along the transmission line. The resistivity or impedance of the thin film is selected to be substantially equal to that of the transformed impedance effected by the cooling liquid. To establish the desired matching transformer, the spacing between the window and the resistive film is selected to be an odd integral multiple of one-fourth the wavelength of a signal as transmitted through the cooling liquid. Further, to ensure that the resistive film absorbs substantially the energy of the signal transmitted along the transmission line, the aforementioned critical distance is selected to be an odd integral multiple of one-fourth the wavelength of the signal as transmitted through the cooling medium. The given dielectric constant of the cooling medium in turn determines the wavelength of a signal transmitted therethrough, and therefore the spacings between the window and the resistive film, and the resistive film and the short circuit.

United States Patent Klein et al.

Primary ExaminerRudolph V. Rolinec Assistant Examiner-Marvin Nussbaum Attorney-F. H. Henson et al.

[5 7] ABSTRACT A termination for terminating or absorbing a signal transmitted along a transmission line is disclosed as comprising a short circuit disposed across the transmission line and a thin film of a resistive material disposed a critical length from the short circuit to absorb substantially the signal transmitted along the transmission line. A cooling chamber is formed about the thin film and a cooling liquid of given dielectric constant and relatively low microwave loss is directed through said cooling chamber to remove the heat absorbed by the thin film. A window is disposed across the transmission line to form the cooling chamber. The cooling liquid of given dielectric constant acts as a matching transformer to reduce the impedance seen by a wave propagated along the transmission line. The resistivity or impedance of the thin film is selected to be substantially equal to that of the transformed impedance effected by the cooling liquid. To establish the desired matching transformer, the spacing between the window and the resistive film is selected to be an odd integral multiple of one-fourth the wavelength of a signal as transmitted through the cooling liquid. Further, to ensure that the resistive film absorbs substantially the energy of the signal transmitted along the transmission line, the aforementioned critical distance is selected to be an odd integral multiple of one-fourth the wavelength of the signal as transmitted through the cooling medium. The given dielectric constant of the cooling medium in turn determines the wavelength of a signal transmitted therethrough, and therefore the spacings between the window and the resistive film, and the resistive film and the short circuit.

9 Claims, 2 Drawing Figures LIQUID DIELECTRIC COOLED TERMINATIONS This invention herein described was made in the course of or under a contract or subcontract thereunder, with the Department of the Air Force.

CROSS REFERENCE TO OTHER APPLICATION Reference is made to co-pending application Ser. No. 189,913, entiltled, Terminations, by Gerald I. Klein, filed Oct. 18, 1971 now abandoned and'assigned to the Assignee of this invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to terminations for transmission lines and in particular to such terminations of reduced size and weight.

2. State of the Prior Art In radar and transmission systems, there arises the need to terminate transmission lines to prevent reflected or standing waves to be established therein. Microwave terminations or dummy loads of the prior art typically employ the use of lossy materials presenting distributed loads along the transmission line for gradual power absorption and impedance matching. Such devices are typically employed for terminating transmission lines carrying rf average power in the range of to 1000 watts. Materials such as graphite and iron loaded epoxies are used in termination loads adapted. for the 10 to 100 watt range dependent upon the frequency and transmission line size desired. For termination loads adapted for higher power dissipation, silicon carbide-type ceramic materials are often employed.

The size and weight of such dummy loads or terminations are critical design criteria where such terminations are to be incorporated into airborne radar systems. As the average power and the frequency of the transmitted wave increase and decrease respectively, the size and the weight of the terminations become larger. For terminations capable of handling lower frequencies and higher average power signals, it may be necessary to further cool the load as by air or liquid coolants.

In the above-identified, co-pending application there is disclosed an improved termination for absorbing a signal transmitted along a transmission line, wherein a short circuit is disposed across the transmission line and a thin film of a resistive material is disposed a critical distance from the short circuit to absorb substantially the energy of the signal transmitted along the transmission line. A heat conductive member is disposed between the thin film and the short circuit having good high-temperature characteristics and capable of efficiently conducting the thermal energy absorbed by the resistive film to the short circuit. The impedance of the thin film is selected to be substantially equal to the characteristic impedance of the transmission line and the aforementioned critical distance is selected to be an odd integral multiple of one-fourth of the wavelength of the signal as transmitted throughthe heat conductive member.

Further, the above-referenced, co-pending application suggests the incorporation of a matching transformer member made of a suitable solid dielectric material disposed infront of the thin film, whereby the impedance as presented to a signal transmitted along the wavelength is reduced. As a result, the impedance or resistivity of the film may also be reduced, whereby the bandwidth characteristic of the termination is broadened significantly.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a new and improved termination capable of absorbing signals of increased power levels.

It is a more specific object of this invention to provide a termination for a transmission line of relatively samll weight and size capable of terminatinghigh levels of power and yet capable of achieving a VSWR in the order of 1.05 to 1.20 over 10-20 percent frequency bands.

The present invention achieves the above-mentioned and additional objects by providing a termination for a transmission line comprising a short circuit, a thin film of a selected resistivity and a cooling chamber for receiving the thin film. The cooling chamber comprises a window disposed across the transmission line whereby a cooling fluid of selected dielectric constant and relatively low microwave loss is directed through a space formed between the window and the resistive film and a space formed between the resistive film and the short circuit. Thus, the cooling medium serves the dual purpose of first, removing the heat absorbed by the thin film and second, acting as an impedance transformer whereby the impedance seen by a signal transmitted along the transmission line is reduced. As a result, the resistivity of the thin film, which is set to equal substantially the transformed impedance seen by a signal transmitted along the transmission line, is reduced and the bandwidth characteristics of the termination improved. Further, the spacings between the resistive film and the short circuit and the resistive film and the window are set to be substantially N )t g/4 where N is any odd integral multiple and Ag is the wavelength of a signal transmitted through the cooling fluid; these aforementioned critical spacings are therefore dependent upon the selected dielectric constant of the cooling fluid.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the present invention will become more apparent by referring to the following detailed description and accompanying drawings, in which:

FIG. 1 is a sectioned, side view of a termination in accordance with teachings of this invention; and

FIG. 2 is a sectioned, top view of the termination of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Wave energy absorption has been discussed in Fields and Waves in Modern Radio, by S. Rama and J. Whinnery, at page 313 (.I. Wiley, 1944), wherein it is stated that a planar wave transmitted in space may be completely absorbed by directing the planar wave normally onto a thin conducting film placed one/quarter wavelength in front of a perfect conductor. To achieve efficient absorption, the sheet resistance of the-thin film as determined by the ratio of the bulk resistivity (p) of the film material to the thickness of the film is set equal to the wave impedance. The ratio of bulk resistivity to thickness (p/T) defines the sheet resistance of the thin film in ohms per square. A thin film 38 as shown in the drawings is placed in a plane at the point where the voltage of the transmitted and reflected waves sum to a maximum to achieve the desired energy absorption. The use of the thin film 38 as opposed to a distributed load achieves a significant reduction in size and weight of this invention. It is understood that such a thin film requires support and additionally would be destroyed by the absorbed energy if not dissipated.

In accordance with the teachings of this invention, a cooling liquid of selected dielectric constant is directed about the thin film 38 to dissipate the absorbed energy and to provide a practical embodiment of this invention capable of handling high levels of signal power. In particular, a termination of this invention comprises a transmission line or waveguide 12 illustratively having a rectangular configuration. The waveguide 12 transmits the signal therealong to the thin film 38 whereat the signal is absorbed substantially. The waveguide 12 is terminated in a short circuit formed by an end member 22 secured to the waveguide 12 as by brazing and made of a suitable electrically conductive material such as aluminum. Illustratively, the end member 22 is made of the same metal as the waveguide 12.

Significantly, the thin film is supported within a cooling chamber defined by a microwave pressure window 14, the walls of the waveguide 12 and the end member 22. The section of the cooling chamber disposed between the window 14 and the thin film 38 is identified by the numeral 24. As illustrated in the drawings, the thin film 38 is supported upon a substrate 38 made of a suitable material such as beryllium oxide having the characteristics of good high-temperature resistance and being inert chemically to the cooling liquid directed through the cooling chamber. The section of the cooling chamber disposed between the substrate 34 and the end member 22 is identified by the numeral 26. As will be explained later, the cooling liquid is pumped into the cooling chamber to flow through both sections 24 and 26 in contact with both sides of the assembly 34 comprising the substrate 36 and the thin film 38. As seen in the drawings, a portion of the waveguide 12 adjacent the end member 22 is made of increased thickness to provide an edge 23 against which the assembly 34 is supported. Illustratively, the assembly 34 is retained against the edge 23 by a suitable adhesive 25 such as that epoxy cement H54 made by Epoxy Technology of Watertown, Massachusetts.

As shown in FIG. 2, the cooling liquid is directed into the cooling chamber by a pump 48 which directs the cooling liquid through a conduit 44 to an entrance opening 40 of the cooling chamber. The cooling fluid is returned from the cooling chamber through an exit opening 42 along a conduit 46 to a heat exchanger comprising, for example, cooling fins or a suitable refrigeration mechanism. The cooled liquid is returned to the pump 48 to be recirculated through the cooling chamber.

The window 14 is inserted across the cross-section of the waveguide 12 to confine the cooling liquid within the aforedescribed cooling chamber and is made of a suitable material such as Teflon fiberglass to permit the transmission therethrough of the signal. As seen in the drawings, the window 14 is mounted within a fail safe"mounting generally designated by the number 16. The mounting 16 includes first and second gaskets 18 and disposed on opposite sides of the window 14. Gasket 20 forms the fluid seal and gasket 18 is a waveguide air pressure seal. lllustratively, the gaskets l8 and 20 may take the form of elastomer O-rings and are disposed within ridges formed within the mounting 16 to resiliently abut the surfaces of the window 14 to prevent leakage of the cooling liquid from the aforedescribed chamber and further, to prevent any cooling fluid from leaking into the forward portion of the waveguide 12. The elastomer O-rings have been successfully employed with cooling liquids operated at high fluid pressure in excess of PSI. A significant feature of the mountings 16 is the provision of a series of openings 30 and 32 disposed about the periphery of the mountings 16 whereby any fluid that may inadvertently leak through the gasket 20 is discharged therethrough and will not be directed to the other side of the window 14.

The use of a single gasket disposed about the edge of the window 14 for both fluid and air seals has been rejected because the cooling fluid may leak into the forward portion of the waveguide to interrupt the transmission of the signal therealong.

The surface of the thin film 38 is spaced a critical distance from the interior surface of the end member 22 comprising the short circuit across the waveguide 12. In particular, the front surface of the thin film 38 is disposed N )tg/4 from the interior surface of the end member 22 to absorb substantially the energy of the signal transmitted along the waveguide 12, where )\g is the wavelength of the signal transmitted through the cooling liquid and N is any odd integral number. The thickness (7') of the film 38 is made sufficiently thin with regard to the wavelength (Ag) of the transmitted signal to ensure that the absorbing effect of the film 38 is concentrated at the point within the waveguide 12 of maximum voltage. This condition is met where:

21m x T 1 Significantly, as will be explained in detail later, the thickness T determines the sheet resistance of the thin film 38 to equal substantially the impedance as seen by the transmitted wave. More specifically, the sheet resistagqe of the th in film 38 is determined by the ratio of its bulkiresistivity p to the film thickness TT In an ill us trativeembodiment of this invention, the film 38 is made of Nichrome to a thickness of approximately 100A to provide a resistivity 108 ohms per square (p of Nichrome being 108 micro-ohms-cm.). Significantly, the thin film 38 is disposed upon the substrate 36 made of a highly thermally conductive material such as beryllium oxide BeO and having a thickness of 0.025 inches. Illustratively, the thin film 38 may be formed by vacuum depositing a layer of Nichrome upon the substrate 36. The thin film 38 may be made of other resistive materials such as thg ermet resistive inks which are compositions of metals, oxide powders and glass suspended in a suitable vehicle and which may be screen printed and fired in air at high temperature to form the thin film onto the substrate 36, or carbon which could be vacuum or pyrolytically deposited upon the substrate 36. W, The incorporation of the cooling fluid of low microwave loss and a given dielectric constant serves a dual function in accordance with the teachings of this invention. First, as explained above, the cooling fluid serves toremove the heat absorbed by the thin film 38. Se-

condly, the cooling fluid as directed within the section 24 between the window 14 and the exposed surface of the thin film 38 serves as a matching transformer whereby the impedance seen by the signal transmitted along the waveguide 12 is reduced and the bandwidth characteristics of the termination are improved.

The impedance that is seen by a signal transmitted along the front portion of the waveguide 12 is the characteristic impedance of that waveguide or transmission line. The characteristic impedance of a transmission line is an inherent characteristic of that transmission line dependent upon the configuation and materials of the transmission line and is not influenced by impediments disposed therein. The characteristic impedance of a transmission line is defined as the impedance of an infinite length of that transmission line having no variation in impedance along its entire length. As mentioned above, the resistivity of the thin film 38 is determined to be substantially equal to that impedance as seen by the transmitted signal at the position of the thin film 38. Significantly, the cooling fluid serves as a matching transformer whereby the impedance seen by the transmitted signal is reduced from the characteristic impedance to a transformed impedance so that the resistivity of the thin film 38 is reduced and the bandwidth characteristics of the termination are improved.

In order to provide the desired transformer characeristic, the spacing between the front surface of the window l4 and the exposed surface of the thin film 38 is selected to be N )tg/4, where Ag is the wavelength of the signal in the cooling liquid of given dielectric constant. When the cooling liquid is directed into the section 24, a matching transformer is formed whereby the transformed impedance Z at the exposed surface of the resistive film 38 is given by the following equation:

where Z is equal to the characteristic impedance of the air-filled waveguide 12 and Z is the characteristic impedance or dielectric constant of the cooling liquid directed through the section 24 of the cooling chamber. Thus, the resistivity or impedance Z of the thin film 38 is made substantially equal to that of the transformed impedance Z as defined by the equation above. As a result, the resistivity of the film is decreased significantly and the bandwidth characteristics of the resultant termination improved.

As explained above, the spacings between the front surface of the thin film 38 and the inner surface of the end member 22, and the exposed surface of the thin film 38 and the front surface of 'the window 14 are set to be substantially equal to NAg/4 where N is an odd integral mutiple and Ag is the wavelength of a signal transmitted through the cooling medium. The wavelength ).g is dependent upon the dielectric constant of the cooling liquid. Thus, the microwave design, including the aforementioned spacings, is in turn dependent upon the given constant of the cooling liquid. In an illustrative embodiment of this invention, the cooling liquid may be selected to be a flourocarbon as manufactured by the Minnesota Mining Company under the designation Flourinert types FC75, 77, etc., or a silicate ester as manufactured by Monsanto under the designation Coolanol, types 15, 20, 25, 45, etc. In determing these critical spacings, the dielectric constants of the substrate 36 and the window 14 are also taken into account. Thus, it is desired to use materials for the window. 14 and the substrate 36 whose characteristic impedance are similar to that of the cooling liquid, so that the aforementioned critical spacings are primarily dependent upon the dielectric constant of the cooling liquid.

Further, the cooling liquid to be circulated through the cooling chamber is selected to have a relatively low microwave loss. Microwave loss is measured in terms of a materials loss tangent. It is contemplated that the cooling liquid of this invention has a low tangent not greater than 0.05, whereas water, which would not be a suitable liquid for this invention, has a tangent loss, dependent upon the frequency of the signal applied thereto and its temperature, many orders of magnitude greater than that of the cooling liquid suitable for this invention. As mentioned above, a cooling liquid such as a Flourinert RG77" is suitable for this invention and has a loss tangent in the order of 0.007.

In one illustrative method of operation, the pump 48 is of such capacity to achieve a flow rate of the cooling liquid on one gallon per minute; such a rate of flow was sufiicient to dissipate 1,500 watts. Further, an S-band, one-pound embodiment has been implemented including a WR229 waveguide, which has been operated to absorb 1,500 watts with a VSWR that did not change significantly from from the low power measurements. The advantage of using the cooling liquid as described herein with regard to the termination as described in the above-referenced, co-pending application is that a significantly higher power handling capability is achieved; it is estimated that the increased power capability of the termination as described herein is in the order of three to four times that of the termination described in the above-identified, co-pending application, thus permitting signals to be terminated having power densities considerably greater than 1 kilowatt per square inch.

Numerous changes may be made in the abovedescribed apparatus and the different embodiments of the invention may be made without departing from the spirit thereof; therefore, it is intended that all matter contained in the foregoing description and in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for absorbing energy from a signal transmitted along a transmision line having a characteristic impedance, said apparatus comprising:

a. short circuit means disposed across said transmission line;

b. a film disposed across substantially the entire cross-section of said transmission line;

0. means for forming a cooling chamber including a first section disposed between said film and said short circuit means and a second section in front of said film with respect to said short circuit means; and a cooling liquid disposed within said cooling chamber and having a given dielectric constant and low microwave loss;

e. the distances along the length of said transmission line from said film to said short circuit means and from said film to the periphery of said cooling chamber being determined to be N x /4, where N is any odd integral number and Ag is the wavelength of a signal transmitted in said cooling liquid;

f. said cooling liquid in said second section of said cooling chamber transforming the impedance seen by the transmitted signal along said transmission line, said film having an impedance substantially equal to the reduced impedance of said transmission line as transformed by said cooling liquid.

2. Apparatus as claimed in claim 1, wherein there is included means for directing said cooling liquid through said cooling chamber.

3. Apparatus as claimed in claim 2, wherein said directing means includes pump means for circulating said cooling liquid through a first conduit to said cooling chamber and a second conduit from said cooling chamber to said pump means.

4. Apparatus as claimed in claim 3, wherein said directing means includes heat exchanger means for dissipating the absorbed heat from said cooling liquid.

5. Apparatus as claimed in claim 1, wherein said cooling chamber defining means includes a window disposed substantially across the cross-section of said transmission line.

6. Apparatus as claimed in claim 5, wherein there is included a first O-ring disposed to form a liquid-tight seal between a first surface of said window and said second section of said cooling chamber and a second 0- ring disposed to form an air and liquid-tight seal between a second surface of said window and the remaining portion of said transmission line.

7. Apparatus as claimed in claim 6, wherein there is included means for supporting said window with respect to said transmission line, said supporting means includes a plurality of openings disposed therethrough to permit cooling liquid inadvertently leaked through said first O-ring to be discharged from said supporting means.

8. Apparatus as claimed in claim 1, wherein said film is supported upon a substrate within said cooling chamber.

9. Apparatus as' claimed in claim 1, wherein said cooling liquid has a loss tangent not greater than 0.05. 

1. Apparatus for absorbing energy from a signal transmitted along a transmission line having a characteristic impedance, said apparatus comprising: a. short circuit means disposed across said transmission line; b. a film disposed across substantially the entire cross-section of said transmission line; c. means for forming a cooling chamber including a first section disposed between said film and said short circuit means and a second section in front of said film with respect to said short circuit means; and d. a cooling liquid disposed within said cooling chamber and having a given dielectric constant and low microwave loss; e. the distances along the length of said transmission line from said film to said short circuit means and from said film to the periphery of said cooling chamber being determined to be N lambda g/4, where N is any odd integral number and lambda g is the wavelength of a signal transmitted in said cooling liquid; f. said cooling liquid in said second section of said cooling chamber transforming the impedance seen by the transmitted signal along said transmission line, said film having an impedance substantially equal to the reduced impedance of said transmission line as transformed by said cooling liquid.
 2. Apparatus as claimed in claim 1, wherein there is included means for directing said cooling liquid through said cooling chamber.
 3. Apparatus as claimed in claim 2, wherein said directing means includes pump means for circulating said cooling liquid through a first conduit to said cooling chamber and a second conduit from said cooling chamber to said pump means.
 4. Apparatus as claimed in claim 3, wherein said directing means includes heat exchanger means for dissipating the absorbed heat from said cooling liquid.
 5. Apparatus as claimed in claim 1, wherein said cooling chamber defining means includes a window disposed substantially across the cross-section of said transmission line.
 6. Apparatus as claimed in claim 5, wherein there is included a first O-ring disposed to form a liquid-tight seal between a first surface of said window and said second section of said cooling chamber and a second 0-ring dispoSed to form an air and liquid-tight seal between a second surface of said window and the remaining portion of said transmission line.
 7. Apparatus as claimed in claim 6, wherein there is included means for supporting said window with respect to said transmission line, said supporting means includes a plurality of openings disposed therethrough to permit cooling liquid inadvertently leaked through said first O-ring to be discharged from said supporting means.
 8. Apparatus as claimed in claim 1, wherein said film is supported upon a substrate within said cooling chamber.
 9. Apparatus as claimed in claim 1, wherein said cooling liquid has a loss tangent not greater than 0.05. 